1. Field of the Invention
This invention relates to the field of ultra-wideband communication systems. More particularly, it relates to the controlled transmission and reception of ultra-wideband electromagnetic pulses.
2. Background of Related Art
Ultra-wideband (UWB) systems, both for radar and communications applications, have historically utilized impulse, or shock-excited, transmitter techniques in which an ultra-short duration pulse (typically tens of picoseconds to a few nanoseconds in duration) is directly applied to an antenna which then radiates its characteristic impulse response. For this reason, UWB systems have often been referred to as “impulse” radar or communications. In addition, since the excitation pulse is not a modulated or filtered waveform, such systems have also been termed “carrier-free” in that no apparent carrier frequency is evident from the resulting RF spectrum.
To be useful for data communications, previous UWB impulse or carrier-free transmission systems have been limited to ON-OFF keying (binary amplitude shift keying ASK) or pulse position modulation (PPM) since amplitude and/or phase control of the waveform was extremely difficult or impossible to implement. In addition, these previous systems have been fixed bandwidth and fixed frequency with no capability for frequency hopping or dynamic bandwidth control.
Output power and pulse repetition frequency (PRF) of UWB impulse transmitters have also been limited due to fundamental physical limitations of the devices used to generate the ultra-short duration pulses. In particular, high output power and high PRF were mutually exclusive properties of such systems. High output power impulse excitation sources such as bulk avalanche semiconductors, high voltage breakover devices, high voltage Gallium Arsenide (GaAs) thyristors, plasma diodes, stacked arrays of step recovery diodes (SRDs), etc. required hundreds to many thousands of volts for proper operation and, consequently, were limited to PRFs below a few tens of kilohertz due to increased device heating and thermal breakdown at higher PRFs. Lower power devices, such as avalanche transistors, low voltage SRDs, Zener diodes, etc., can operate at PRFs of several megahertz, but produced output powers many orders of magnitude lower. In addition, while the individual devices were typically low cost, they often needed to be hand-selected in order to guarantee avalanche or breakdown characteristics at a particular operating voltage level.
As an example, early versions of UWB impulse transmitters typically generated less than one watt peak microwave output power at a maximum PRF of approximately 10 kHz using baseband impulse excitation powers of tens to several thousand watts. Several laboratory models using these high voltage sources were constructed for radar applications which included ship docking, pre-collision sensing for automobiles, liquid level sensing, and intrusion detection. Although these techniques proved to be reliable, the power efficiency, PRF limitations, size and complicated antenna assemblies limited performance and reproducibility.
Another significant limitation of such impulse-based UWB sources is the fact that the power level decreases with increasing frequency at a rate of approximately 12 dB per octave. This is due to the double exponential nature of the impulse excitation. The output response from a typical impulse source has the form:
      p    ⁡          (      t      )        =            t      α        ⁢          e              (                  1          -                      t            α                          )              ⁢                  u                  -          1                    ⁡              (        t        )            where α is the pulse rise time and u-1(t) is the unit step function. FIG. 10 shows the output response p(t) versus time. This waveform closely approximates the output seen from the vast majority of impulse sources.
One can now compute the instantaneous pulse power versus frequency (magnitude-squared Fourier transform) as:
      P    ⁡          (      f      )        =                    e        2                    16        ⁢                  π          4                      ⁢          1                        α          2                ⁢                  f          4                    
Note that if the rise time is doubled, the power at any given frequency decreases by 6 dB. Similarly, for a constant peak voltage source, doubling the frequency of operation decreases the output power by 12 dB.
As an example, a 2.5 kW peak power output thyristor-based impulse generator develops only about 1 watt peak power at L-Band (1.5 GHz range) since the vast majority of the impulse energy is produced at significantly lower frequencies. This unused energy is dissipated as heat, subjecting operating circuits to overheating and damage, and limiting the PRF or data rate at which the source can operate reliably. The upper trace in FIG. 11 shows the rapid drop in available power versus frequency from a conventional thyristor-based impulse source.
Another limitation in the use of such techniques is the lack of accurate control of radiated emissions to meet regulatory requirements. Since a short pulse excitation will stimulate the impulse response of an antenna, and a typical wideband antenna has a frequency response extending over many octaves in frequency (an octave of frequency being a doubling of frequency), the radiated spectrum will be extremely broadband, covering hundreds of megahertz (MHz) to several gigahertz (GHz) or more of instantaneous bandwidth. This broad spectrum may overlap many frequencies of operation licensed otherwise by the U.S. Federal Communications Commission (FCC) in the U.S. or by other means in foreign countries, thus presenting a concern to operators or users of allocated frequencies, albeit at very low average power levels.
Thus, conventional UWB signal generation techniques suffer from several shortcomings:                (i) high power operation can only be achieved at reduced PRFs because of device heating;        (ii) practical operational frequencies are limited to well below 5 GHz due to the 12 dB per octave falloff of output impulse energy with increased frequency;        (iii) impulse excitation of an antenna results in a “carrier-free” signal which would uncontrollably overlap frequencies restricted from such use, albeit with low energy densities; and        (iv) modulation techniques are limited to on-off keying and pulse position modulation, with no capability for frequency hopping or for dynamic bandwidth control.        
There is a need to achieve a higher output power for long distance communications and for small target detection in the case of a radar system, to develop high PRFs for the transmission of wideband video and data, to produce UWB transmissions at well-controlled center frequencies and bandwidths extending to higher operating frequencies (e.g., millimeter wave), and to allow for newer and more efficient modulation techniques.
One of the first ultra wideband (previously referred to as baseband, carrier-free or short pulse) receivers was patented in 1972 by Ken Robbins while at the Sperry Research Center, U.S. Pat. No. 3,662,316. This receiver utilized a “dispersionless” broadband transmission line antenna together with a biased tunnel diode located in the transmission line for detecting the total energy in a pulse and expanding the resultant output in the time domain so that conventional, lower speed circuitry may be used for processing. The tunnel diode was biased to operate as a monostable multivibrator as disclosed in 1962 in Gentile, S. P., Basic Theory and Application of Tunnel Diodes, Van Nostrand, NJ, ch. 8 “Pulse and Switching Circuits” (1962). The receiver took advantage of the tunnel diode's unique characteristic of changing state when the area under the current vs. time envelope, i.e., the charge carriers passing through the device, exceeded a prescribed number of picocoulombs. This change in state yielded a recognizable, detectable event or output voltage. Sperry's tunnel diode detector (TDD) receiver was used in a number of applications including baseband communications, liquid level sensing, object detection and radar. It was soon observed, however, that the Robbins TDD was subject to operating point bias drift due to temperature and power supply fluctuations. This bias drift impacted negatively the system's overall sensitivity and increased the false alarm rate.
In 1976, Nicolson and Mara introduced a constant false alarm rate (CFAR) circuit to the tunnel diode detector receiver that is described in U.S. Pat. No. 3,983,422. The CFAR circuit employed a logic circuit that sampled noise dwells and data dwells to dynamically adjust a variable avalanche threshold of the tunnel diode. This feedback circuit operated in such a manner that the false alarm rate, as measured by the number of hits received due solely to noise during a fixed time interval, was held constant regardless of temperature fluctuations, power supply voltage changes, device aging, etc. The CFAR receiver was utilized in the development of baseband speed sensing, collision avoidance, and radar docking prototypes.
In 1987, an anti-jam circuit was introduced into the CFAR receiver. This is described in U.S. Pat. No. 4,688,041. Since the baseband receiver was extremely broadband, with typical bandwidths of hundreds of MHz to GHz, it was found to be extremely susceptible to in-band interference and jamming since the tunnel diode circuit could not distinguish between valid and unwanted signals. Such in-band signals caused a significant reduction in receiver sensitivity by causing the CFAR loop to back-off the sensitivity of the tunnel diode detector. The anti-jam circuit disclosed in U.S. Pat. No. 4,688,041 used the jamming signal itself (if sufficiently strong), or else an internally switched continuous wave (CW) signal, as a local oscillator signal to heterodyne the incoming signal prior to detection. However, this anti-jam circuit proved to be ineffective in the presence of barrage (broadband) noise, jamming or interference, and/or multiple in-band CW interfences. In the case of barrage noise, no reference frequency is provided by the interference with which to down convert the incoming signal, and the system reverts to single-conversion super heterodyne operation with an internal first local oscillator. The broadband noise is also down converted with the signal, and no anti-jam improvement is obtained. In the latter case of multiple in-band CW interferers, the circuitry will use one of these tones, or a linear combination depending upon the third order intercept properties of the design. In this case, the remaining tones are also heterodyned to near baseband and act once again as strong in-band jamming signals.
Also in 1987, U.S. Pat. No. 4,695,752 disclosed a narrow range gate added to the existing baseband CFAR receiver. The reduction in range gate size had the effect of reducing unwanted noise and interference by more closely matching the detector with the received pulse duration. The inventor of this patent purports to achieve nanosecond range gate intervals through the use of two Germanium (Ge) and a single Gallium Arsenide (GaAs) tunnel diode.
In 1994, U.S. Pat. No. 5,337,054 to Ross and Mara disclosed a coherent processing tunnel diode UWB receiver. These inventors claim to have improved tunnel diode detector receiver sensitivity by using a tunnel diode envelope generator to perform a super heterodyne conversion whereby the available charge for triggering the tunnel diode is maximized. Ross and Mara considered only single pulse ultra wideband detectors; i.e., detectors which make a binary, or hard, decision (Logic 1 or Logic 0) at every sampling instant. However, their patent discloses a sliding average of detector hits, noise dwell or data dwell, in any group of thirty-two consecutive periods (col. 4, lines 35–39). Averaging of all hits, including data dwells, provides an average of the noise dwells which is skewed because of the inclusion of the data swells. Moreover, to reduce the effects of the skewing, a large number of noise dwells must be detected for each data dwell detected, ultimately reducing data rates.
There have been other patented UWB receiver designs in which a multiplicity of pulses (typically several thousand) are first coherently added, or integrated, before a binary (bit) decision is made (e.g., U.S. Pat. Nos. 5,523,760; 4,979,186; and 5,363,108). The UWB detectors of the present invention do not require coherent addition of a multiplicity of pulses, but rather have sufficient sensitivity to operate on a single pulse basis.
Only false alarm rate is typically computed by the processor of an UWB receiver, and thus the system bit error rate (BER), and accordingly the receiver operating characteristic (ROC) are unknown. In practice, the tunnel diode bias is “backed off” from the CFAR level to reduce the BER to an acceptable level. Unfortunately, since the BER is a very sensitive function of the tunnel diode bias level, this can result in a significant reduction in receiver sensitivity to achieve a desired BER.
As disclosed in U.S. Pat. No. 3,662,316, in a tunnel diode UWB receiver, the tunnel diode changes state whenever the accumulated charge on the device exceeds a given threshold. Mathematically, the performance of the tunnel diode detector in additive white Gaussian noise (AWGN) can be described by the following set of equations:
            P      d        =                  Probability        ⁡                  (                                                    max                                  0                  ≤                  t                  ≤                  T                                            ⁢                                                ∫                  0                  t                                ⁢                                                      (                                                                  s                        ⁡                                                  (                          u                          )                                                                    +                                                                        n                          w                                                ⁡                                                  (                          u                          )                                                                                      )                                    ⁢                                      ⅆ                    u                                                                        ≥                          T              h                                )                    ⁢                          ⁢      and                  P      fa        =          Probability      ⁡              (                                            max                              0                ≤                t                ≤                T                                      ⁢                                          ∫                0                t                            ⁢                                                                    n                    w                                    ⁡                                      (                    u                    )                                                  ⁢                                  ⅆ                  u                                                              ≥                      T            h                          )            where Pd is the probability of detection, Pfa is the probability of false alarm, s(u) is the received UWB waveform, nw(u) is additive white Gaussian noise with double-sided power spectral density N0B, B is the detection signal bandwidth, T is the diode dwell sensitivity interval, and Th is a threshold value.
While previous designs of the CFAR tunnel diode receiver have functioned reliably as an ultra wideband single pulse detector, their use in modern communication and radar applications have presented numerous drawbacks:                1. The prior art designs remain susceptible to in-band interference and jamming, particularly broadband or barrage noise jamming and multiple CW interferers.        2. The requirement to continuously adjust bias to the tunnel detector to maintain a given constant false alarm rate (CFAR) conventionally requires a minimum number of noise dwells to take place for each data dwell—typically thirty-two or more noise dwells for each data dwell—to achieve false alarm rates less than a few percent. This severely restricts the maximum data rate at which a single detector can operate since data and noise dwells must operate at different time intervals. In addition, the speed at which the tunnel diode detector can respond to sudden changes in the electromagnetic environment is limited. Hence, impulsive noise (which is nearly always present) can create burst errors in the data stream, corrupting data integrity.        3. Receiver sensitivity is conventionally backed-off to achieve a desired BER, providing an UWB receiver which has reduced distance capability and slower data rates.        